This study is aimed at elucidating the regulation of fatty acid oxidation in heart muscle and at establishing the mode by which the interrelated pathways of beta-oxidation and ketone body degradation affect each other. As part of this study, an ongoing investigation of the enzymes of beta-oxidation will be completed. The regulation of fatty acid oxidation will be studied by evaluating the possible regulatory function of a recently isolated mitochondrial thioesterase from pig heart, by determining the rate-limiting step(s) of beta-oxidation and by assessing the effects of mitochondrial metabolites and coenzymes as well as the possible covalent modification of enzymes on the activities of the beta-oxidation enzymes. Special attention will be given to the elucidation of possible regulatory mechanisms of 3-ketoacyl-CoA thiolase and aceto-acetyl-CoA thiolase which are central enzymes in beta-oxidation and ketone body degradation respectively and which may be instrumental in the preferential utilization of fatty acids vs. ketone bodies observed in muscle under certain conditions. An important part of this project is a detailed study of the multi-enzyme complex of fatty acid oxidation from E. coli. Results from work on the E. coli complex are expected to aid an investigation of the intramitochondrial organization of the enzymes of beta-oxidation which will be initiated by performing exploratory experiments aimed at demonstrating specific affinities of the enzymes of beta-oxidation for each other.